Pheromone-regulated Sumoylation of Transcription Factors That Mediate the Invasive to Mating Developmental Switch in Yeast

Depolymerization of SUMO chains induces slender to stumpy differentiation in T. brucei bloodstream parasites

Trypanosoma brucei are protozoan parasites that cause sleeping sickness in humans and nagana in cattle. Inside the mammalian host, a quorum sensing-like mechanism coordinates its differentiation from a slender replicative form into a quiescent stumpy form, limiting growth and activating metabolic pathways that are beneficial to the parasite in the insect host. The post-translational modification of proteins with the Small Ubiquitin-like MOdifier (SUMO) enables dynamic regulation of cellular metabolism. SUMO can be conjugated to its targets as a monomer but can also form oligomeric chains. Here, we have investigated the role of SUMO chains in T. brucei by abolishing the ability of SUMO to polymerize. We have found that parasites able to conjugate only SUMO monomers are primed for differentiation. This was demonstrated for monomorphic lines that are normally unable to produce stumpy forms in response to quorum sensing signaling in mice, and also for pleomorphic cell lines in which stumpy cells were observed at unusually low parasitemia levels. SUMO chain mutants showed a stumpy compatible transcriptional profile and better competence to differentiate into procyclics. Our study indicates that SUMO depolymerization may represent a coordinated signal triggered during stumpy activation program.

Whole Transcriptome Analysis Provides Insights Into the Molecular Mechanisms of Chlamydospore-Like Cell Formation in Phanerochaete chrysosporium

Phanerochaete chrysosporium is a white rot fungus naturally isolated from hardwoods and widely used in environmental pollution control because it produces extracellular peroxidases. It forms chlamydospores during nitrogen starvation, which naturally occurs in the habitat of P. chrysosporium. Chlamydospores protect fungi against many stresses; the molecular basis underlying chlamydospore formation in basidiomycetes is poorly explored. Chlamydospores in P. chrysosporium have a different cell wall compared with hyphae, as confirmed by cell wall digestion and microscopy. Furthermore, this study investigated the transcriptome of P. chrysosporium in different life stages, including conidium, hypha, and chlamydospore formation, through RNA sequencing. A total of 2215 differentially expressed genes were identified during these processes. The expression patterns of genes involved in several molecular events critical for chlamydospore formation, including starch and sucrose metabolism, phosphatase and kinase, and transcription factors, were determined. This study serves as a basis for further investigating the function of chlamydospore formation in the biotechnologically relevant fungus P. chrysosporium.

A Chemical and Enzymatic Approach to Study Site-Specific Sumoylation

A variety of cellular pathways are regulated by protein modifications with ubiquitin-family proteins. SUMO, the Small Ubiquitin-like MOdifier, is covalently attached to lysine on target proteins via a cascade reaction catalyzed by E1, E2, and E3 enzymes. A major barrier to understanding the diverse regulatory roles of SUMO has been a lack of suitable methods to identify protein sumoylation sites. Here we developed a mass-spectrometry (MS) based approach combining chemical and enzymatic modifications to identify sumoylation sites. We applied this method to analyze the auto-sumoylation of the E1 enzyme in vitro and compared it to the GG-remnant method using Smt3-I96R as a substrate. We further examined the effect of smt3-I96R mutation in vivo and performed a proteome-wide analysis of protein sumoylation sites in Saccharomyces cerevisiae. To validate these findings, we confirmed several sumoylation sites of Aos1 and Uba2 in vivo. Together, these results demonstrate that our chemical and enzymatic method for identifying protein sumoylation sites provides a useful tool and that a combination of methods allows a detailed analysis of protein sumoylation sites.

Pheromone-induced morphogenesis and gradient tracking are dependent on the MAPK Fus3 binding to Gα

Unique roles are found for the MAPK Fus3 during the mating response of yeast. In particular, the interaction of Fus3 with the G-protein α-subunit is required for morphogenesis and gradient tracking and suppresses cell-to-cell variability between mating and chemotropic fates in a population of pheromone-responding cells.

Mitogen-activated protein kinase (MAPK) pathways control many cellular processes, including differentiation and proliferation. These pathways commonly activate MAPK isoforms that have redundant or overlapping function. However, recent studies have revealed circumstances in which MAPK isoforms have specialized, nonoverlapping roles in differentiation. The mechanisms that underlie this specialization are not well understood. To address this question, we sought to establish regulatory mechanisms that are unique to the MAPK Fus3 in pheromone-induced mating and chemotropic fate transitions of the budding yeast Saccharomyces cerevisiae. Our investigations reveal a previously unappreciated role for inactive Fus3 as a potent negative regulator of pheromone-induced chemotropism. We show that this inhibitory role is dependent on inactive Fus3 binding to the α-subunit of the heterotrimeric G-protein. Further analysis revealed that the binding of catalytically active Fus3 to the G-protein is required for gradient tracking and serves to suppress cell-to-cell variability between mating and chemotropic fates in a population of pheromone-responding cells.

SUMOylation is developmentally regulated and required for cell pairing during conjugation in Tetrahymena thermophila

The covalent attachment of small ubiquitin-like modifier (SUMO) to target proteins regulates numerous nuclear events in eukaryotes, including transcription, mitosis and meiosis, and DNA repair. Despite extensive interest in nuclear pathways within the field of ciliate molecular biology, there have been no investigations of the SUMO pathway in Tetrahymena. The developmental program of sexual reproduction of this organism includes cell pairing, micronuclear meiosis, and the formation of a new somatic macronucleus. We identified the Tetrahymena thermophila SMT3 (SUMO) and UBA2 (SUMO-activating enzyme) genes and demonstrated that the corresponding green fluorescent protein (GFP) tagged gene products are found predominantly in the somatic macronucleus during vegetative growth. Use of an anti-Smt3p antibody to perform immunoblot assays with whole-cell lysates during conjugation revealed a large increase in SUMOylation that peaked during formation of the new macronucleus. Immunofluorescence using the same antibody showed that the increase was localized primarily within the new macronucleus. To initiate functional analysis of the SUMO pathway, we created germ line knockout cell lines for both the SMT3 and UBA2 genes and found both are essential for cell viability. Conditional Smt3p and Uba2p cell lines were constructed by incorporation of the cadmium-inducible metallothionein promoter. Withdrawal of cadmium resulted in reduced cell growth and increased sensitivity to DNA-damaging agents. Interestingly, Smt3p and Uba2p conditional cell lines were unable to pair during sexual reproduction in the absence of cadmium, consistent with a function early in conjugation. Our studies are consistent with multiple roles for SUMOylation in Tetrahymena, including a dynamic regulation associated with the sexual life cycle.

Regulating global sumoylation by a MAP kinase Hog1 and its potential role in osmo-tolerance in yeast

Sumoylation, a post-translational protein modification by small ubiquitin-like modifier (SUMO), has been implicated in many stress responses. Here we analyzed the potential role of sumoylation in osmo-response in yeast. We find that osmotic stress induces rapid accumulation of sumoylated species in normal yeast cells. Interestingly, disruption of MAP kinase Hog1 leads to a much higher level of accumulation of sumoylated conjugates that are independent of new protein synthesis. We also find that the accumulation of sumoylated species is dependent on a SUMO ligase Siz1. Notably, overexpression of SIZ1 in HOG1-disruption mutants (hog1Δ) but not in wild type cells leads to a markedly increased and prolonged accumulation of sumoylated species. Examination of osmo-tolerance of yeast mutants that display either an increase or a decrease in the global sumoylation level revealed an inverse relationship between accumulation of sumoylated conjugates and osmo-tolerance. Further investigation has shown that many of the sumoylated species induced by hyperosmotic stress are actually poly-sumoylated. Together, these findings indicate that abnormal accumulation of poly-sumoylated conjugates is harmful for osmo-tolerance in yeast, and suggest that Hog1 promotes adaptation to hyperosmotic stress partially via regulation of global sumoylation level.

Dynamic ubiquitination of the mitogen-activated protein kinase kinase (MAPKK) Ste7 determines mitogen-activated protein kinase (MAPK) specificity

Ubiquitination is a post-translational modification that tags proteins for proteasomal degradation. In addition, there is a growing appreciation that ubiquitination can influence protein activity and localization. Ste7 is a prototype MAPKK in yeast that participates in both the pheromone signaling and nutrient deprivation/invasive growth pathways. We have shown previously that Ste7 is ubiquitinated upon pheromone stimulation. Here, we show that the Skp1/Cullin/F-box ubiquitin ligase SCF(Cdc4) and the ubiquitin protease Ubp3 regulate Ste7 ubiquitination and signal specificity. Using purified components, we demonstrate that SCF(Cdc4) ubiquitinates Ste7 directly. Using gene deletion mutants, we show that SCF(Cdc4) and Ubp3 have opposing effects on Ste7 ubiquitination. Although SCF(Cdc4) is necessary for proper activation of the pheromone MAPK Fus3, Ubp3 is needed to limit activation of the invasive growth MAPK Kss1. Finally, we show that Fus3 phosphorylates Ubp3 directly and that phosphorylation of Ubp3 is necessary to limit Kss1 activation. These results reveal a feedback loop wherein one MAPK limits the ubiquitination of an upstream MAPKK and thereby prevents spurious activation of a second competing MAPK.

Proper protein glycosylation promotes mitogen-activated protein kinase signal fidelity

The ability of cells to sense and respond appropriately to changing environmental conditions is often mediated by signal transduction pathways that employ mitogen-activated protein kinases (MAPKs). In the yeast Saccharomyces cerevisiae, the high-osmolarity glycerol (HOG) and filamentous growth (FG) pathways are activated following hyperosmotic stress and nutrient deprivation, respectively. Whereas the HOG pathway requires the MAPK Hog1, the FG pathway employs the MAPK Kss1. We conducted a comprehensive screen of nearly 5000 gene deletion strains for mutants that exhibit inappropriate cross-talk between the HOG and FG pathways. We identified two novel mutants, mnn10Δ and mnn11Δ, that allow activation of Kss1 under conditions that normally stimulate Hog1. MNN10 and MNN11 encode mannosyltransferases that are part of the N-glycosylation machinery within the Golgi apparatus; deletion of either gene results in N-glycosylated proteins that have shorter mannan chains. Deletion of the cell surface mucin Msb2 suppressed the mnn11Δ phenotype, while mutation of a single glycosylation site within Msb2 was sufficient to confer inappropriate activation of Kss1 by salt stress. These findings reveal new components of the N-glycosylation machinery needed to ensure MAPK signaling fidelity.

Combined computational and experimental analysis reveals mitogen-activated protein kinase-mediated feedback phosphorylation as a mechanism for signaling specificity

A series of mathematical models was used to quantitatively characterize pheromone-stimulated kinase activation and determine how mitogen-activated protein (MAP) kinase specificity is achieved. The findings reveal how feedback phosphorylation of a common pathway component can limit the activity of a competing MAP kinase through feedback phosphorylation of a common activator, and thereby promote signal fidelity.

Different environmental stimuli often use the same set of signaling proteins to achieve very different physiological outcomes. The mating and invasive growth pathways in yeast each employ a mitogen-activated protein (MAP) kinase cascade that includes Ste20, Ste11, and Ste7. Whereas proper mating requires Ste7 activation of the MAP kinase Fus3, invasive growth requires activation of the alternate MAP kinase Kss1. To determine how MAP kinase specificity is achieved, we used a series of mathematical models to quantitatively characterize pheromone-stimulated kinase activation. In accordance with the computational analysis, MAP kinase feedback phosphorylation of Ste7 results in diminished activation of Kss1, but not Fus3. These findings reveal how feedback phosphorylation of a common pathway component can limit the activity of a competing MAP kinase through feedback phosphorylation of a common activator, and thereby promote signal fidelity.

The TEA transcription factor Tec1 links TOR and MAPK pathways to coordinate yeast development

In Saccharomyces cerevisiae, the TEA transcription factor Tec1 controls several developmental programs in response to nutrients and pheromones. Tec1 is targeted by the pheromone-responsive Fus3/Kss1 mitogen-activated protein kinase (MAPK) cascade, which destabilizes the transcription factor to ensure efficient mating of sexual partner cells. The regulation of Tec1 by signaling pathways that control cell division and development in response to nutrients, however, is not known. Here, we show that Tec1 protein stability is under control of the nutrient-sensitive target of rapamycin complex 1 (TORC1) signaling pathway via the Tip41-Tap42-Sit4 branch. We further show that degradation of Tec1 upon inhibition of TORC1 by rapamycin does not involve polyubiquitylation and appears to be proteasome independent. However, rapamycin-induced Tec1 degradation depends on the HECT ubiquitin ligase Rsp5, which physically interacts with Tec1 via conserved PxY motives. We further demonstrate that rapamycin and mating pheromone control Tec1 protein stability through distinct mechanisms by targeting different domains of the transcription factor. Finally, we show that Tec1 is a positive regulator of yeast chronological lifespan (CLS), a known TORC1-regulated process. Our findings indicate that in yeast, Tec1 links TORC1 and MAPK signaling pathways to coordinate control of cellular development in response to different stimuli.

Sumoylation of transcription factor Tec1 regulates signaling of mitogen-activated protein kinase pathways in yeast

Tec1 is a transcription factor in the yeast mitogen-activated protein kinase (MAPK) pathway that controls invasive growth. Previously we reported that a fraction of Tec1 protein is sumoylated on residue lysine 54 in normally growing cells. Here we describe regulation and functional consequences of Tec1 sumoylation. We found that activation of Kss1, the MAPK that directly activates Tec1, results in a decrease in Tec1 sumoylation and a concurrent increase of Tec1 transcriptional activity. Consistent with a role of sumoylation in inhibiting Tec1 activity, specifically increasing sumoylation of Tec1 by fusing it to the sumoylating enzyme Ubc9 leads to a dramatic decrease of Tec1 transcriptional activity. Invasive growth is also compromised in Tec1-Ubc9. In contrast, fusing sumoylation-site mutant Tec1, i.e., Tec1^K54R, to Ubc9 did not significantly alter transcriptional activation and had a less effect on invasive growth. Taken together, these findings provide evidence for regulated sumoylation as a mechanism to modulate the activity of Tec1 and validate Ubc9 fusion-directed sumoylation as a useful approach for studying protein sumoylation.

Mechanisms and constraints on yeast MAPK signaling specificity

The survival of cells relies on their ability to respond specifically to diverse environmental signals. Surprisingly, intracellular signaling pathways often share the same or homologous protein components, yet undesirable crosstalk is, in general, suppressed. This signaling specificity has been well studied in the yeast model system Saccharomyces cerevisiae, where the mitogen-activated protein kinase (MAPK) cascades are repeatedly employed in mediating distinct biological processes including pheromone-induced mating and filamentous growth under starvation. Although various mechanisms have been proposed to interpret the yeast MAPK signaling specificity, a consistent theory is still lacking. Here, we present a mathematical model that shows signaling specificity can arise through asymmetric hierarchical inhibition. The parameters of our model are, where possible, based on experimental data that allow us to determine the constraints imposed by signaling specificity on these parameters. Our model is in broad agreement with experimental observations to date and generates testable predictions that may stimulate further research.

Efficient yeast ChIP-Seq using multiplex short-read DNA sequencing

BACKGROUND

Short-read high-throughput DNA sequencing technologies provide new tools to answer biological questions. However, high cost and low throughput limit their widespread use, particularly in organisms with smaller genomes such as S. cerevisiae. Although ChIP-Seq in mammalian cell lines is replacing array-based ChIP-chip as the standard for transcription factor binding studies, ChIP-Seq in yeast is still underutilized compared to ChIP-chip. We developed a multiplex barcoding system that allows simultaneous sequencing and analysis of multiple samples using Illumina's platform. We applied this method to analyze the chromosomal distributions of three yeast DNA binding proteins (Ste12, Cse4 and RNA PolII) and a reference sample (input DNA) in a single experiment and demonstrate its utility for rapid and accurate results at reduced costs.

RESULTS

We developed a barcoding ChIP-Seq method for the concurrent analysis of transcription factor binding sites in yeast. Our multiplex strategy generated high quality data that was indistinguishable from data obtained with non-barcoded libraries. None of the barcoded adapters induced differences relative to a non-barcoded adapter when applied to the same DNA sample. We used this method to map the binding sites for Cse4, Ste12 and Pol II throughout the yeast genome and we found 148 binding targets for Cse4, 823 targets for Ste12 and 2508 targets for PolII. Cse4 was strongly bound to all yeast centromeres as expected and the remaining non-centromeric targets correspond to highly expressed genes in rich media. The presence of Cse4 non-centromeric binding sites was not reported previously.

CONCLUSION

We designed a multiplex short-read DNA sequencing method to perform efficient ChIP-Seq in yeast and other small genome model organisms. This method produces accurate results with higher throughput and reduced cost. Given constant improvements in high-throughput sequencing technologies, increasing multiplexing will be possible to further decrease costs per sample and to accelerate the completion of large consortium projects such as modENCODE.

A novel Bayesian DNA motif comparison method for clustering and retrieval

Characterizing the DNA-binding specificities of transcription factors is a key problem in computational biology that has been addressed by multiple algorithms. These usually take as input sequences that are putatively bound by the same factor and output one or more DNA motifs. A common practice is to apply several such algorithms simultaneously to improve coverage at the price of redundancy. In interpreting such results, two tasks are crucial: clustering of redundant motifs, and attributing the motifs to transcription factors by retrieval of similar motifs from previously characterized motif libraries. Both tasks inherently involve motif comparison. Here we present a novel method for comparing and merging motifs, based on Bayesian probabilistic principles. This method takes into account both the similarity in positional nucleotide distributions of the two motifs and their dissimilarity to the background distribution. We demonstrate the use of the new comparison method as a basis for motif clustering and retrieval procedures, and compare it to several commonly used alternatives. Our results show that the new method outperforms other available methods in accuracy and sensitivity. We incorporated the resulting motif clustering and retrieval procedures in a large-scale automated pipeline for analyzing DNA motifs. This pipeline integrates the results of various DNA motif discovery algorithms and automatically merges redundant motifs from multiple training sets into a coherent annotated library of motifs. Application of this pipeline to recent genome-wide transcription factor location data in S. cerevisiae successfully identified DNA motifs in a manner that is as good as semi-automated analysis reported in the literature. Moreover, we show how this analysis elucidates the mechanisms of condition-specific preferences of transcription factors.

Regulation of gene expression plays a central role in the activity of living cells and in their response to internal (e.g., cell division) or external (e.g., stress) stimuli. Key players in determining gene-specific regulation are transcription factors that bind sequence-specific sites on the DNA, modulating the expression of nearby genes. To understand the regulatory program of the cell, we need to identify these transcription factors, when they act, and on which genes. Transcription regulatory maps can be assembled by computational analysis of experimental data, by discovering the DNA recognition sequences (motifs) of transcription factors and their occurrences along the genome. Such an analysis usually results in a large number of overlapping motifs. To reconstruct regulatory maps, it is crucial to combine similar motifs and to relate them to transcription factors. To this end we developed an accurate fully-automated method, termed BLiC, based upon an improved similarity measure for comparing DNA motifs. By applying it to genome-wide data in yeast, we identified the DNA motifs of transcription factors and their putative target genes. Finally, we analyze motifs of transcription factor that alter their target genes under different conditions, and show how cells adjust their regulatory program in response to environmental changes.

Sumoylation in Aspergillus nidulans: sumO inactivation, overexpression and live-cell imaging

Sumoylation, the reversible covalent attachment of small ubiquitin-like modifier (SUMO) peptides has emerged as an important regulator of target protein function. In Saccharomyces cerevisiae, but not in Schizosaccharyomes pombe, deletion of the gene encoding SUMO peptides is lethal. We have characterized the SUMO-encoding gene, sumO, in the filamentous fungus Aspergillus nidulans. The sumO gene was deleted in a diploid and sumODelta haploids were recovered. The mutant was viable but exhibited impaired growth, reduced conidiation and self-sterility. Overexpression of epitope-tagged SumO peptides revealed multiple sumoylation targets in A. nidulans and SumO overexpression resulted in greatly increased levels of protein sumoylation without obvious phenotypic consequences. Using five-piece fusion PCR, we generated a gfp-sumO fusion gene expressed from the sumO promoter for live-cell imaging of GFP-SumO and GFP-SumO-conjugated proteins. Localization of GFP-SumO is dynamic, accumulating in punctate spots within the nucleus during interphase, lost at the onset of mitosis and re-accumulating during telophase.

Regulation of gene transcription by mitogen-activated protein kinase signaling pathways

Mitogen-activated protein kinase (MAPK) signaling pathways are key mediators of eukaryotic transcriptional responses to extracellular signals. These pathways control gene expression in a number of ways including the phosphorylation and regulation of transcription factors, co-regulatory proteins and chromatin proteins. MAPK pathways therefore target multiple components of transcriptional complexes at gene promoters and can regulate DNA binding, protein stability, cellular localization, transactivation or repression, and nucleosome structure. Recent work has uncovered further complexities in the mechanisms by which MAPKs control gene expression including their roles as integral components of transcription factor complexes and their interplay with other post-translational modification pathways. In this review I discuss these advances with particular focus on how MAPK signals are integrated by transcription factor complexes to provide specific transcriptional responses and how this relates to cellular function.

Function and regulation in MAPK signaling pathways: lessons learned from the yeast Saccharomyces cerevisiae

Signaling pathways that activate different mitogen-activated protein kinases (MAPKs) elicit many of the responses that are evoked in cells by changes in certain environmental conditions and upon exposure to a variety of hormonal and other stimuli. These pathways were first elucidated in the unicellular eukaryote Saccharomyces cerevisiae (budding yeast). Studies of MAPK pathways in this organism continue to be especially informative in revealing the molecular mechanisms by which MAPK cascades operate, propagate signals, modulate cellular processes, and are controlled by regulatory factors both internal to and external to the pathways. Here we highlight recent advances and new insights about MAPK-based signaling that have been made through studies in yeast, which provide lessons directly applicable to, and that enhance our understanding of, MAPK-mediated signaling in mammalian cells.

The RACK1 ortholog Asc1 functions as a G-protein beta subunit coupled to glucose responsiveness in yeast

According to the prevailing paradigm, G-proteins are composed of three subunits, an alpha subunit with GTPase activity and a tightly associated betagamma subunit complex. In the yeast Saccharomyces cerevisiae there are two known Galpha proteins (Gpa1 and Gpa2) but only one Gbetagamma, which binds only to Gpa1. Here we show that the yeast ortholog of RACK1 (receptor for activated protein kinase C1) Asc1 functions as the Gbeta for Gpa2. As with other known Gbeta proteins, Asc1 has a 7-WD domain structure, interacts directly with the Galpha in a guanine nucleotide-dependent manner, and inhibits Galpha guanine nucleotide exchange activity. In addition, Asc1 binds to the effector enzyme adenylyl cyclase (Cyr1), and diminishes the production of cAMP in response to glucose stimulation. Thus, whereas Gpa2 promotes glucose signaling through elevated production of cAMP, Asc1 has opposing effects on these same processes. Our findings reveal the existence of an unusual Gbeta subunit, one having multiple functions within the cell in addition to serving as a signal transducer for cell surface receptors and intracellular effectors.

Mathematical models of specificity in cell signaling

Cellular signaling pathways transduce extracellular signals into appropriate responses. These pathways are typically interconnected to form networks, often with different pathways sharing similar or identical components. A consequence of this connectedness is the potential for cross talk, some of which may be undesirable. Indeed, experimental evidence indicates that cells have evolved insulating mechanisms to partially suppress "leaking" between pathways. Here we characterize mathematical models of simple signaling networks and obtain exact analytical expressions for two measures of cross talk called specificity and fidelity. The performance of several insulating mechanisms--combinatorial signaling, compartmentalization, the inhibition of one pathway by another, and the selective activation of scaffold proteins--is evaluated with respect to the trade-off between the specificity they provide and the constraints they place on the network. The effects of noise are also examined. The insights gained from this analysis are applied to understanding specificity in the yeast mating and invasive growth MAP kinase signaling network.

Pheromone-induced degradation of Ste12 contributes to signal attenuation and the specificity of developmental fate

The Ste12 transcription factor of Saccharomyces cerevisiae regulates transcription programs controlling two different developmental fates. One is differentiation into a mating-competent form that occurs in response to mating pheromone. The other is the transition to a filamentous-growth form that occurs in response to nutrient deprivation. These two distinct roles for Ste12 make it a focus for studies into regulatory mechanisms that impart biological specificity. The transient signal characteristic of mating differentiation led us to test the hypothesis that regulation of Ste12 turnover might contribute to attenuation of the mating-specific transcription program and restrict activation of the filamentation program. We show that prolonged pheromone induction leads to ubiquitin-mediated destabilization and decreased amounts of Ste12. This depletion in pheromone-stimulated cultures is dependent on the mating-pathway-dedicated mitogen-activated protein kinase Fus3 and its target Cdc28 inhibitor, Far1. Attenuation of pheromone-induced mating-specific gene transcription (FUS1) temporally correlates with Ste12 depletion. This attenuation is abrogated in the deletion backgrounds (fus3Delta or far1Delta) where Ste12 is found to persist. Additionally, pheromone induces haploid invasion and filamentous-like growth instead of mating differentiation when Ste12 levels remain high. These observations indicate that loss of Ste12 reinforces the adaptive response to pheromone and contributes to the curtailing of a filamentation response.